1. Field of the Invention
This invention relates to a rotary fluid displacement apparatus, and more particularly, to an improvement in a rotation preventing/thrust bearing device for an orbiting member fluid displacement apparatus.
2. Description of the Prior Art
There are several types of fluid apparatus which utilize an orbiting piston or fluid displacing me, tuber, such as a scroll type fluid displacement apparatus disclosed in U.S. Pat. No. 801,182 to Creux.
The scroll type fluid displacement apparatus disclosed in this U.S. patent includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the,, two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Because the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is capable of compressing, expanding, or pumping fluids.
Generally, in conventional scroll type fluid displacement apparatus, one scroll is fixed to a housing and the other scroll, which is the orbiting scroll, is eccentrically supported on a crank pin of a rotating shaft to cause the orbital motion. The scroll type fluid displacement apparatus also includes a rotation preventing device which prevents the rotation of the orbiting scroll to thereby maintain both scrolls in a predetermined angular relationship during operation of the apparatus.
Sealing along the line contacts of the above conventional scroll type apparatus must be maintained because the fluid pockets are defined by the line contacts between the two spiral elements and as the line contacts shift along the surface of the spiral elements, the fluid pockets change volume due to the orbital motion of the orbiting scroll. Because the orbiting scroll in such conventional scroll type apparatus is supported in a cantilever manner, an axial slant of the orbiting scroll occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbiting motion caused by eccentric movement of a crank pin driven by the rotation of a drive shaft. Several problems result from the axial slant; such as, loss of sealing of the line contact, vibration of the apparatus during operation, and noise caused by physical striking of the spiral elements.
One simple and direct solution to this problem is the use of a thrust bearing device for carrying the axial thrust load. Thus, scroll type fluid displacement apparatus have been provided with rotation preventing and thrust bearing devices within their housings.
One recent attempt to improve rotation preventing and thrust bearing devices for scroll type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 to Hidden et al. and 4,259,043 to Hidden et al. The rotation preventing and thrust bearing devices in these U.S. patents are integral with one another. The rotation preventing/thrust bearing device described in these U.S. patents (see, e.g., FIG. 7 of U.S. Pat. No. 4,259,043 to Hidden et al.), comprises one set of indentations formed on the end surface of the circular plate of the orbiting scroll and a second set of indentations formed on an end surface of a fixed plate attached to the housing. A plurality of spheres are placed between facing indentations. Nevertheless, the indentations are formed directly on the end surface of orbiting scroll or the fixed plate. The production of this type of mechanism is, therefore, very intricate.
Referring to FIGS. 1, 2, and 3, one solution to the above disadvantage will be described. FIG. 1 is an enlarged vertical section view of a part of a compressor and FIG. 2 is an exploded perspective view of a rotation preventing/thrust bearing device 37'. Rotation preventing/thrust bearing device 37' surrounds boss 273 of orbiting scroll 27. Annular steps 274', 275, and 276, which are concentrically surrounding boss 273, are formed at the end surface of circular end plate 271 opposite to spiral element 272. Annular step 274' is radially largest and closest to spiral element 272. Annular step 276 is radially smallest and furthest from spiral element 272. Annular step 275 is located between annular steps 274' and 276. Similarly, annular steps 113' and 115 are formed at the end surface of annular projection 112 of front end plate 11, which rotatably supports a drive shaft (not shown) and is fixedly attached to an opening end of casing 12. Annular steps 113' and 115 are concentric with annular projection 112, and annular step 113' is radially smallest and furthest from spiral element 272.
Rotation preventing/thrust bearing device 37' includes an orbital portion, a fixed portion and bearings, such as a plurality of balls or spheres. The fixed portion includes (1) first annular race 371 which is disposed surrounding annular step 113' by a later-mentioned manner and (2) first ring 372 fitted against the axial end surface of annular projection 112 of front end plate 11 to cover the end surface of first annular race 371. First annular race 371 is loosely fitted within annular step 113' because the outer diameter of first annular race 371 is designed to be slightly smaller than a diameter of an annular side wall 113'a of annular step 113'. First ring 372 is fixedly attached to the axial end surface of annular projection 112 by pins 373. First annular race 371 has an axial end surface flush with the axial end surface of annular step 115. The height differential between the axial end surface of annular step 115 and the axial end surface of annular projection 112 of front end plate 11 defines a clearance "G" between first annular race 371 and first ring 372.
The orbital portion includes (1) second annular race 374 which is disposed within annular step 274' by a later-mentioned manner and (2) second ring 375 fitted against the axial end surface of annular step 276 to cover the axial end surface of second annular race 374. Second annular race 374 is loosely fitted within annular step 274' because an inner diameter of second annular race 374 is designed to be slightly greater than a diameter of an annular side wall 274' a of annular step 274'. Second ring 375 is fixedly attached to the axial end surface of annular step 276 by pins 376. Second annular race 2;74 has an axial end surface flush with the axial end surface of annular step 275. The height differential between the axial end surface of annular step 275 and the axial end surface of annular step 276 defines a clearance "G" between the second annular race 374 and the second ring 375 identical to the clearance between the first annular race 371 and the first ring 372.
First ring 372 and second ring 375 each have a plurality of holes or pockets 372a and 375a in the axial direction, the number of holes or pockets in each ring 372, 375 being equal. The holes or pockets 372a of first ring 372 correspond to or are a mirror image of the holes or pockets 375a of the second ring 375, i.e., each pair of pockets facing each other have the same size and pitch, and the radial distance of the pockets from the center of their respective rings 372 and 375 is the same, i.e., the centers of the pockets are located the same distance from the center of the rings 372 and 375. Bearing elements, such as balls or spheres 377, are placed between facing, generally aligned pairs of pockets 372a and 375a.
Referring to FIG. 3, the operation of the rotation preventing/thrust bearing device 37' will be described. In FIG. 3, the center of second ring 375 is placed at the right side and the rotating direction of the drive shaft is clockwise, as indicated by arrow "A." When orbiting scroll 27 is driven by the rotation of the drive shaft, the center of second ring 375 orbits about a circle of radius "R.sub.o " (together with orbiting scroll 27). Nevertheless, a rotating force, i.e., moment, which is caused by the offset of the acting point of the reaction force of compression and the acting point of drive force, acts on orbiting scroll 27. This reaction force tends to rotate orbiting scroll 27 in a clockwise direction about the center of second ring 375. As shown in FIG. 3, however, eighteen balls 377 are placed between the corresponding pockets 372a and 375a of rings 372 and 375. In Figure .3, the interaction between the nine balls 377 at the top of the rotation preventing/thrust bearing device and the edges of the pockets 372a and 375a prevents the rotation of orbiting scroll 27. The magnitude of the rotation preventing forces are shown as fc.sub.1 -fc.sub.5 in FIG. 3. According to the orbital motion of orbiting scroll 27, the interaction between the nine balls 377 and the edges of the pockets 372a and 375a successively shifts in the rotating direction of the drive shaft.
Not only does the reaction force of compression tend to rotate orbiting scroll in the clockwise direction, but it tends to move orbiting scroll 27 forwardly (to the left in FIG. 1) to thereby cause the axial thrust load on an inner end of the drive shaft through bushing 34. This axial thrust load is carried by the front end plate 11 through second annular race 374, all eighteen balls 377 and first annular race 371. Therefore, each of eighteen balls 377 comes in contact with the end surface of both first and second annular races 371 and 374, and rolls thereon within the corresponding pockets 372a and 375a during the orbital motion of orbiting scroll 27. As balls 377 roll on the axial end surface of first annular race 371, the first annular race 371 freely rotates on the axial end surface of the annular step 113' because of a frictional contact between balls 377 and race 371. As a result, the circular trace of the balls 377 on the axial end surface of first annular race 371 is sufficiently dissolved so that the exfoliation of the axial end surface of first annular race 371 is effectively prevented. Similarly, the second annular race 374 freely rotates on the axial end surface of annular step 274' in the same rotational direction, so that a similar advantage to that described above is also obtained.
In the construction, as described above, the rotation preventing/thrust bearing device 37' is made up of a pair of races and a pair of rings, with each race and ring formed separately. Therefore, the parts of the rotation/thrust bearing device are easy to construct and the most suitable material for each part can be selected. In general, in order to be able to sufficiently bear the axial thrust load and the interacting stress, balls 377, first and second rings 372 and 375, and first and second annular races 371 and 374 are made of stiff and hard material, for example, steel; while in order to reduce the weight of the compressor, front end plate 11, casing 12, and the two scroll members are made of light weight material, for example, aluminum alloy, which is relatively softer than steel.
Accordingly, as first annular race 371 freely rotates on the axial end surface of the annular step 113' of front end plate 11 during operation of the compressor, the axial end surface of first annular race 371 and the axial end surface of annular step 113' become in a frictional contact between hard and soft metals. This frictional contact causes an abnormal abrasion at the axial end surface of annular step 113'. Therefore, the clearance "G" between first annular race 371 and first ring 372 becomes sufficiently greater than that allowable in a short time period during operation of the compressor, and a similar defective operational manner also occurs between the second annular race 374 and second ring 375. As a result, the compressor begins to defectively operate in a short time period.